High Temperature Superconductors (HTS) are type II superconductors that react in a known manner when exposed to applied magnetic fields. When an HTS is exposed to a magnetic field >Hcl at approximately 20 mT, the magnetic field enters the superconductor as a self-interacting lattice of quantized line vortices or fluxons. Each fluxon is a tube of radius London penetration depth λ(T) with superconducting current circulating around a non-superconducting core of radius on the order of the coherence length ζ(T). The magnetic field trapped in each fluxon is quantized with a value 2×10−15 Wb.
To carry a stable current, the fluxon lattice in the superconductor must be prevented from moving by pinning each fluxon in an area that contains a non-superconducting material or defect. The size of pinning defects should be on the order of the coherence length, approximately 1-2 nm at 4.2K and 2-4 nm at 77K. The introduction of any type of intrinsic or extrinsic defect can act as effective pinning center. Introduction of extrinsic defects provide a straightforward method to control flux pinning, without disturbing the superconductor.
Extrinsic defects have been added by a variety of processes which typically fall into two categories: irradiation or bombarding the HTS films with energetic particles which displaces material leaving tracks, or by incorporating small inclusions of alternate material into the films. Irradiation is considered too expensive to be practical.
The desirability of providing efficient high temperature superconductors for operation at 20° K and higher is well known. Indeed, there has been an enormous amount of experimental activity in these so called high temperature superconductors since research in the mid 1980s first demonstrated dramatic gains in raising the maximum critical transition temperatures from the 20° K range to the 90° K range.
In general, superconductors and superconducting material exhibit zero resistance when operating at temperatures below their maximum critical transition temperature. This quality of operating at zero resistance facilitates the construction and operation of highly efficient devices such as superconducting magnets, magnetic levitators, propulsion motors and magnetohydronamics, power generators, particle accelerators, microwave and infrared detectors, etc.
High temperature superconducting (HTS) generators and magnets are significantly lighter and more compact than their conventional counterparts. The development of these devices is essential to military applications requiring compact, lightweight, high power sources or compact high field magnets, especially ground mobile, airborne and naval applications. The high temperature superconducting coated conductor can be used to make the coil windings in HTS generators as well as the HTS magnet windings. As such, long lengths of the YBCO coated conductor with high current transport in a magnetic field are necessary for effective use in these applications.
Given the important role high temperature superconductors will play in future technology, several attempts have been made to improve high temperature superconductors. For example, Calcium (Ca) doping at Y site in Y123 films has been studied extensively, as Ca is expected to increase the population of holes in Y123 by replacing Y+3 with Ca+2 hence improving superconductor coupling between the grains (intergranularly). Scientists have reported a 2 to 6 fold improvement of critical current density (Jc) between large-angle grains (intergranular Jc) for annealed Y123/Ca-Y123/Y123 multi-layer films at 77 K and lower temperatures, however only for very large angle grain boundaries 24° [001] ab in-plane misorientation. Other scientists have reported that. Ca doping increased the intergranular critical current density in spite of a lower critical transition temperature (Tc) in higher concentrations of Ca doped Y123 films; however such increases were only measured well below Tc at about 60K or less.
In U.S. Pat. No. 6,830,776 (along with other references), Barnes et al. demonstrated that Y123/Y211 nanoparticulates/Y123 multi-layers exhibit highest magnetization Jc due to uniform distribution of nano-ordered pinning centers.
There is a further need for low-cost methods for incorporating inclusions or defects into HTS. The present invention addresses this need by providing 1) minute quantities of rare earth elements or other deleterious elements in YBCO thin films and 2) providing a combination effect of Ca doping as well as 211 pinning in Y123 materials to increase the critical current carrying capability across grain boundaries as well as in high magnetic fields.